This invention relates generally to optical signal processing systems and more particularly to beamforming controls for phased array antennas in radar systems.
Phased array antenna systems employ a plurality of individual antenna elements or subarrays of antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative bean from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antenna elements (or subarrays of antennas) can be accomplished by electronically shifting the phases of the actuating signals or by introducing a time delay in the different actuating signals to sequentially excite the antenna elements to generate the desired direction of beam transmission from the antenna. Electronically shifting the phases of a large number of actuating signals, such as is required in large sophisticated phased-array radars, requires extensive equipment, including switching devices to route the electrical signals through appropriate hardwired circuits to achieve the desired phase changes, and has numerous operational limitations which are drawbacks in a phased array system using broad band radiation.
Optical control systems, however, can be advantageously used to create selected time delays in actuating signals for phased array systems. Such optically generated time delays are not frequency dependent and thus can be readily applied to broadband phased array antenna systems. For example, optical signals can be processed to establish the selected time delays between individual signals to cause the desired sequential actuation of the transmitting antenna elements, and the optical signals can then be converted to electrical signals, such as by a photosensor array. Different optical architectures have been proposed to process optical signals to generate selected delays, such as routing the optical signal through optical fiber segments of different lengths or utilizing free space propagation based delay lines, which architecture typically incorporates polarizing beam splitters and prisms. Performance of both types of optical delay systems is a function, among other things, of the rapidity with which optical switching is accomplished. In fiber based systems, several optical switches have been suggested, for example lithium niobate electro-optic waveguide based cross-bar switches, electrically switched multiple semiconductor laser-based switches, and MESFET-based gallium arsenide 1.times.2 switches connected in a back-to-back configuration to implement a 2.times.2 electrical switch that implements optical switching using several semiconductor lasers. All of these switch systems are impractical for use in a large phased array antenna, e.g., an antenna having 1000 or more antenna elements, due to the high insertion loss, high crosstalk level, and high cost of the switches.
An optical beam forming system for a phased array antenna that avoids the above drawbacks is disclosed in the copending application of N. Riza entitled "Reversible Time Delay Beamforming Optical Architecture for Phased Array Antennas," Ser. No. 07/690,421, filed Apr. 24, 1991, allowed Dec. 18, 1991, and which is assigned to the assignee of the present invention and incorporated herein by reference. The optical control system disclosed in the above referenced application is a transmit/receive phased array beamformer for generating true-time-delays using optical free-space delay lines and two dimensional liquid crystal spatial light modulators for implementing the optical switching. Unlike the switching techniques mentioned earlier, the liquid crystal-based optical switching elements can provide low insertion loss and low crosstalk level switching with relatively easily fabricated and low cost liquid crystals. Liquid crystal-based optical switches, however, have relatively slow switch response times that limit the scanning speed of a phased array antenna.
High performance phased array radars preferably are able to scan several hundred beams per second while having a relatively long detection range. To achieve such performance it is important that the radar have a sufficiently long dwell time, i.e., the period when the array is transmitting or receiving along a given beam path, to provide the desired range capability, and have a minimum of dead time, i.e., the finite time it takes to reset the beamforming controls for a new beam direction during which the radar is not transmitting or receiving. Longer dead times necessitate that either the number of beams that can be scanned per second be limited or that the dwell time of each be limited; both of these limitations adversely affect radar performance, limiting range, the probability of detecting a target, and the rate at which target information is updated. Dead time thus preferably constitutes a very small percentage of the radar's dwell time. For example, in advanced conventional phased array radars using digital phase shifters controlling over 4000 antenna elements in an array, the percentage of dead time versus dwell time is about 0.2%, which corresponds to 200 scans or transmit/receive sequences per second having a dwell time per beam of about 5 msec, which corresponds to a maximum unambiguous range of 750 km, and a 10 .mu.sec dead time between successive transmit/receive sequences.
The switching time for arrays using liquid crystal optical switches can range from tens of milliseconds to a few microseconds. Nematic liquid crystals switch in a few milliseconds using control voltages of about 3-5 volts, but have been shown to have switching times of about 100 .mu.sec when control voltages of about 50 volts are used. Ferroelectric liquid crystals have demonstrated switching times of 10-100 .mu.sec under control voltages of about 30-50 volts. Nematic liquid crystals are, however, more readily fabricated in large arrays at lower costs, and various thin-film transistor based addressing techniques have been developed for driving the liquid crystal pixels using approximately 5 volt control signals. In addition, nematic liquid crystals have shown up to 4000:1 on/off ratios. Thus, low voltage nematic liquid crystals are desirably used for large area two-dimensional liquid crystal switching arrays, with the key limitation being the several milliseconds switching time.
It is accordingly an object of this invention to provide a fast (a few hundred beams per second) opto-electronic signal control system for a phased array antenna that uses the relatively slow (several milliseconds response time) liquid crystal switching arrays in an optical true-time-delay beamforming architecture.
It is another object of the present invention to provide a fast (a few hundred beams per second) opto-electronic signal control system for a phased array antenna that provides a relatively short radar dead time to increase antenna sensitivity and probability of target detection.
It is another object of the present invention to provide a readily fabricated opto-electronic signal control system for a phased array antenna having a plurality of channels and that has low optical losses, low inter-channel crosstalk, and a relatively short dead time in switching between channels.